This invention relates to a hydraulic tappet-clearance compensating arrangement for cam-controlled valve lifters.
Conventional hydraulic tappet-clearance compensating arrangements, which are used at various locations in actuators for valve lifters for intake and exhaust valves in internal combustion engines, include a check valve which normally has a movable valve body such as a ball with a compression spring acting on it. This conventional check valve design takes up a relatively large amount of space in the tappet-clearance compensating arrangement whereas, for reasons of limited space and for the purpose of reducing the moving masses of the valve actuator, the tappet-clearance compensating arrangement should be especially compact. Even when a departure has been made from the ball valve design for the check valves in hydraulic tappet-clearance compensating arrangements, as, for example, in the arrangement shown in U.S. Pat. No. 3,728,990 by the use of a discoid valve member, the check valve, which is loaded against spring resistance by the pressure of a pressure medium normally diverted from the oil supply of the engine, provides an element of uncertainty. Therefore, a design for a hydraulic tappet-clearance compensating arrangement which does not use such a check valve would be advantageous.
German Patent No. 28 43 918 describes a design for a hydraulic tappet of a valve actuator for an internal combustion engine wherein the check valve is provided by a spring-loaded sealing element which is forcibly actuatable by an additional track designed on the cam. Specifically, this arrangement provides a cup-shaped spring-loaded tappet piston surrounding a reservoir for pressure medium and disposed between two piston-like end pieces which are axially displaceable in an outer cylinder, defining a pressure chamber which accommodates a compression spring between the end of the tappet piston on one side and one of the end pieces on the other. Accordingly, in this arrangement, all of the parts accommodated by the outer cylinder, i.e., the end pieces and tappet piston, are carried within the outer cylinder and are axially displaceable relative thereto. In the wall of the tappet piston and in the outer cylinder, there are provided recesses and through-holes which, on the one hand, establish a connection between the reservoir and the pressure-medium supply at all positions of the outer cylinder with respect to the parts accommodated by it and, on the other hand, ensure a connection between the reservoir and pressure chamber only at a selected relative axial position of the tappet piston with respect to the outer cylinder. This relative axial position between the tappet piston and outer cylinder is established by an additional cam track which displaces the outer cylinder against spring resistance relative to the piston during the base circle phase of the cam. The additional cam track is shaped so that, outside the base circle phase of the cam, the spring resistance displaces the outer cylinder relative to the piston, which is disposed in the path of the transmission of force between cam and valve, into a relative axial position in which the flow connection is interrupted.
As demonstrated by this explanation of the conventional arrangement, it is not very suitable for mass production because of its incorporation of many components which must be movable relative to one another while preserving tightness and because of its complicated cam shape.